It must be emphasized from the outset that the teachings of the present invention are applicable to any horizontal continuous furnace for the annealing or heat treating of a metallic strip and having hearth rolls for the support of the metallic strip as it passes therethrough. In the usual practice, the metallic strip is advanced through such a furnace by tension, the hearth rolls being idler rolls. It is not uncommon, however, for some or all of the hearth rolls to be driven, to assist in advancing the strip through the furnace. Under either circumstance, it is a practical impossibility to synchronize the speed of all of the hearth rolls with that of the metallic strip. As a result, there is unavoidable slippage and rubbing between the hearth rolls and the bottom surface of the strip. This slippage and rubbing, in turn, results in metal transfer from the strip to the surface of the hearth rolls. Furthermore, dirt, oxides and the like are often abraded from the bottom surface of the strip and transferred to the hearth roll surfaces. This contaminate transfer from the strip to the hearth roll, once begun, builds up rapidly. The localized build-up of these contaminates on the hearth roll surfaces eventually result in scratching, denting or dinging of the strip surface. It can, indeed, result in irreparable damage to the strip surface, particularly in those instances where the ultimate use of the metallic strip requires a high finish surface.
Instances where the metallic strip is to be coated with a molten coating metal, this damage to the strip surface can result in an unsatisfactory end product. Furthermore, this contaminant build up on the hearth rolls can be transferred back onto the bottom surface of the strip, resulting in uncoated spots or metal lumps on the strip after the strip has passed through a coating bath.
It has further been found that certain furnace atmospheres and elevated temperatures above about 427.degree. C. will tend to promote and accelerate contamination transfer from the strip to the hearth rolls. Under some circumstances, the contamination-coated hearth rolls may tend to stick to the metallic strip causing further damage to its bottom surface.
Horizontal continuous hearth roll furnaces demonstrating the above noted contamination pick-up problem on their hearth rolls are found in various situations. For example, they are found in metallic coating lines. In the fluxless hot dip metallic coating of steel strip it is necessary to subject the surfaces of the strip to a preliminary treatment which provides a clean surface free of iron oxide scale and other surface contaminants and which is readily wettable by the molten coating metal in order to obtain good adherence. In this country, two types of in-line anneal preliminary treatments are in common use. One is the so-called Sendzimir process (or oxidation-reduction practice) disclosed in U.S. Pat. Nos. 2,110,893 and 2,197,622. The other is the so-called Selas process (or high intensity direct fired furnace line) disclosed in U.S. Pat. Nos. 3,320,085 and 3,720,546.
In accordance with the basic Sendzimir process, steel strip is heated in an oxidizing furnace to a temperature of about 370.degree. C. to about 485.degree. C. without atmosphere control. The strip is withdrawn into air to form a controlled surface oxide layer thereon and is thereafter introduced into a reduction furnace containing a hydrogen-nitrogen atmosphere wherein the strip is heated from about 485.degree. to about 925.degree. C. and the controlled oxide layer is completely reduced. The strip is then passed into a cooling section containing a protective reducing atmosphere and is brought approximately to the temperature of the molten coating metal bath. From the cooling section, the strip is led beneath the bath surface while still surrounded by the protective atmosphere.
In the basic Selas process, steel strip is passed through a direct fired preheat furnace section which has a temperature of about 1315.degree. C. by direct combustion of fuel and air to produce gaseous products of combustion containing at least about 3% combustibles in the form of carbon monoxide, the stock reaching a temperature of from about 425.degree. to about 705.degree. C., while maintaining bright steel surfaces completely free from oxidation. The stock is then passed into a reducing section and from the reducing section to a cooling section wherein it is cooled to a temperature approximating the molten coating metal bath temperature. From the cooling section, the strip is again led beneath the surface of the bath while still surrounded by the protective atmosphere of the cooling section.
U.S. Pat. No. 3,936,543 teaches an improvement in the basic Selas process. U.S. Pat. No. 4,123,291 teaches that a sulfur-bearing coke oven gas can be used as a fuel in the directed fired furnace sections of both the Sendzimir and Selas processes.
As exemplary instances of the problem sought to be overcome by the present invention, in all of the coating lines of the above noted patents wherein a horizontal continuous hearth roll furnace is used in the reducing section, contamination transfer to the hearth rolls can occur in approximately the first third of such furnaces. In the remainder of such furnaces, the tendency of contaminant transfer to the hearth rolls is much less since strip surface contaminants have been removed from the strip during approximately the first one-third of the reducing furnace.
Another well known coating line process is the so-called flux process wherein the strip is pretreated to render its surfaces free of oxide and contaminants and is passed through a flux tank, followed by preheating and passing beneath the surface of a molten coating metal bath. Some of these processes involve a chemical cleaning followed by induction heating to about 427.degree. C. before the strip enters the coating pot. Under these circumstances, where the heating is conducted in a horizontal hearth roll furnace, contamination transfer to the hearth rolls can occur.
Some metallic strip treatment processes entail the use of horizontal continuous hearth roll annealing or normalizing furnaces. Such furnaces are normally used in the production of uncoated strip. Usually, the furnace is open at both ends and uses an oxidizing atmosphere in both the heating and cooling sections. Thereafter, the strip is frequently pickled to remove scale after annealing. Under some circumstances, the entire furnace length (with the possible exception of the entry portion) is subject to hearth roll pick-up. This is true because reducing atmospheres are not used and the contaminants are not removed from the strip.
The above are exemplary instances in which the problem to which the present invention is directed can occur. Contamination transfer to hearth rolls and resultant damage to the metallic strip have long been recognized by the worker in the art. Numerous approaches have been taken to avoid or minimize the problem. For example, U.S. Pat. Nos. 2,279,917 and 4,182,635 teach catenary furnaces wherein hearth rolls are eliminated. Such furnaces, however, are more expensive to build and maintain because they are larger and require more building space. Furthermore, the use of cantenary furnaces is not generally an acceptable solution, if non-catenary furnaces already exists.
Another approach is taught by U.S. Pat. No. 3,643,381 and Japanese application No. 126906/74 published May 8, 1976. U.S. Pat. No. 3,649,381 teaches the provision of perforated hearth rolls supplied with an inert gas to provide a film of gas at the interface between the hearth roll and the metallic strip supported thereby. The Japanese published application teaches the directing of pressurized reducing gas onto a hearth roll surface immediately prior to contact with the metal strip and maintaining a reducing gas atmosphere around the roll. This approach, however, requires specially designed rolls or nozzles, together with feed lines and the like for each hearth roll in the furnace subject to contamination transfer.
The present invention is based upon the discovery that in a horizontal continuous hearth roll furnace for the treatment of metallic strip, contamination transfer to the hearth rolls and resulting damage to the strip surface can be avoided by the provision of at least one appropriately placed, hearth roll which will lift the metallic strip off of the remaining critical hearth rolls subject to contamination transfer. Preferably, the lifting roll is vertically shiftable between a down position in alignment with or below the other hearth rolls and an elevated or lifting position. The provision of a vertically shiftable roll within a furnace is not, per se, new. For example, U.S. Pat. No. 1,956,401 teaches the provision of vertically adjustable rolls within a furnace or at its entrance and exits ends to control the amount of sag of a freely hanging band or wire passing through a straight, horizontal furnace. U.S. Pat. No. 3,284,073 teaches an elongated horizontal furnace for stress relief annealing. The furnace is provided with two sets of rollers, the rollers of each set having two axially adjacent working surfaces. The sets of rollers are shiftable both vertically and transversely of the furnace so that fresh roller working surfaces can be brought into contact with the strip being treated, without shutting down the furnace.